1. Field of the Disclosure
The present disclosure relates to an improved process for making titanium dioxide pigment, and in particular to an improved finishing process for making titanium dioxide pigments.
2. Background of the Disclosure
In producing pigmentary titanium dioxide (TiO2) in rutile form a titanium tetrahalide such as titanium tetrachloride (TiCl4) along with a rutile formation agent such as aluminum trichloride (AlCl3) is reacted in the vapor phase with an oxygen-containing gas in a reactor at a temperature in the range of about 900° C. to 1600° C. to produce a hot gaseous suspension of TiO2 solid particles and free chlorine. This hot gaseous suspension must be quickly cooled below 600° C. within about 1–60 seconds following discharge of the suspension from the reactor. This cooling is accomplished in a conduit, e.g., a flue, which is externally cooled with flowing water so that undesired TiO2 particle size growth is prevented and particle agglomeration is minimized. Particle size and particle agglomeration are important TiO2 pigment properties.
The particle size of the TiO2 pigment can be measured in terms of carbon black undertone (CBU). Pigments containing smaller sized particles have a relatively high CBU, and finished products (e.g., paints, plastics, etc.) containing such pigments tend to have a bluish tint. Pigments with larger sized particles have a relatively low CBU and finished products containing such pigments tend to have a more yellowish tint. The particle agglomeration of the pigment is typically measured in terms of its particle size distribution. Pigments, wherein a low weight percentage of the particles (e.g., less than 30%) have a particle diameter size greater than 0.6 microns, tend to have low particle agglomeration and finished products made with such pigments tend to have high gloss. Pigments, wherein a high weight percentage of the particles have a particle diameter size greater than 0.6 microns, tend to have greater particle agglomeration and finished products made with such pigments tend to have less gloss.
It is known that the production of titanium dioxide pigment may be improved when the TiCl4 and an oxygen-containing gas are reacted in the presence of a nucleant. The method provides TiO2 pigment having improved particle size uniformity, color, and in-process bulk density.
However, in the manufacturing methods described above, the TiO2 particles have a strong tendency to deposit on the inner walls of the cooling conduit. The cooled TiO2 particles tend to form adherent layers on the inner walls and can cause plugging of the conduit. Further, the TiO2 deposits are poor heat conductors and the internal surfaces of the cooling conduit can become insulated which inhibits the heat-exchange properties of the conduit. Scouring material (examples are water-soluble salts like NaCl, KCl, CsCl or insoluble oxides such as TiO2 or SiO2) can be introduced into the cooling conduit to remove the deposits.
The titanium dioxide formed is then subjected to wet treatment, filtration, and drying before the particles are subjected to micronization processes. When the scrubs are water soluble, they dissolve and are removed from the pigment during these processing steps. When they are insoluble, they are separated either by wet screening or by other techniques. A need exists for a streamlined finishing process that eliminates the need for the wet treatment, filtration, and drying steps.
The titanium dioxide product can contain high levels of halogens resulting from upstream processes in which a halogen-containing feedstock, such as titanium tetrahalide is used. The source of halogens in the titanium dioxide product may also be from additives introduced to the process such as one or more halogen-containing rutile-forming agents or nucleants. It is believed that the halides become adsorbed to the surface of the titanium dioxide product, which makes them a challenge to remove. A need exists for a process which is capable of reducing halides from the titanium dioxide product.